The present invention relates generally to iron-aluminum alloys which possess high room-temperature ductility and high resistance to oxidation, aqueous corrosion, and sulfidation. More particularly, the present invention is directed to such Fe-Al alloys which, when wrought and annealed, have ductilities at room temperature of greater than 20%, which are virtually resistant to hydrogen embrittlement, and which possess tensile properties useful in many applications where Fe-Al alloys can be beneficially employed.
Iron-aluminide alloys are of considerable interest for use as a structural material in place of heavier and more expensive stainless steels since Fe-Al alloys possess levels of resistance to oxidation and sulfidation comparable with and often better than many stainless steels. Of the Fe-Al alloys presently known, the Fe-Al alloys with iron and aluminum concentrations in or near Fe.sub.3 Al compositions that have an ordered phase and a lattice structure known as DO.sub.3 at temperatures below about 550.degree. C. have been found to be particularly suitable for use as a structural material in applications requiring relatively high ultimate tensile and yield strength. However, it has been found that presently available iron-aluminide alloys suffer some shortcomings which considerably detract from the use of these alloys as structural materials in many applications. For example, presently available Fe-Al alloys lack sufficient room-temperature ductility to permit the formation of the alloys into desired product configurations at relatively low temperatures. These presently available Fe-Al alloys also suffer a significant loss of strength at temperatures above about 600.degree. C., have relatively low resistance to aqueous corrosion and insufficient resistance to environmental embrittlement, as apparently caused by the dissociation of water molecules in the presence of aluminum atoms on the surface of the alloy for forming alloy-embrittling atomic hydrogen.
Efforts to overcome the aforementioned and other shortcomings of Fe-Al alloys as well as to improve the already present desirable characteristics of the Fe-Al alloys including such alloys in or near the Fe.sub.3 Al compositions have met with varying degrees of success. Many of these efforts have been directed towards the addition of various alloying elements to the binary iron-aluminum alloys for the purpose of improving the ductility and tensile properties of the alloy. For example, as described in U.S. Pat. No. 1,550,508, issued Aug. 18, 1925, the addition of 5 to 10% chromium to a iron-aluminum alloy containing 12 to 16% aluminum was used to enhance the high temperature workability of the alloy. In another example, U.S. Pat. No. 3,026,197, issued Mar. 20, 1962, describes modifying an iron-aluminum alloy containing between 3 and 18% aluminum by adding zirconium and boron to the alloy for the purpose of controlling grain size in the alloy. Also, as described in commonly assigned U.S. Pat. No. 4,961,903, issued Oct. 9, 1990, iron-aluminum alloys with iron and aluminum concentrations based on the Fe.sub.3 Al composition were provided with additions of chromium, molybdenum, niobium, zirconium, vanadium, boron, carbon, and yttrium for increasing the high temperature strength of the alloys and increasing the room-temperature ductility of the alloys from about 2% to 10%. This commonly assigned patent also refers to other prior efforts utilized for the purpose of improving the ductility and tensile properties of iron-aluminum alloys and is incorporated herein by reference.
Other investigations into the Fe-Al alloy system includes a technical article entitled, "An Iron-Aluminum-Molybdenum Alloy as a Chromium-Free Stainless Steel Substitute", by J. S. Dunning, U.S. Department of Interior Report of Investigations No. 8654, (1982) available from the U.S. Government Printing Office (1982-505-002/31). This article describes that ternary iron-aluminum-molybdenum alloys containing 8 nominal weight percent (up to 7.62 actual weight percent) aluminum and 6 weight percent molybdenum can be provided with additions of zirconium and carbon for providing a dispersed phase of zirconium carbide to strengthen the solid solution matrix. The molybdenum was used in the alloy for solid solution strengthening purposes. These alloys were reported as having a room-temperature elongation of up to 18%, ultimate tensile strengths up to about 99 ksi, and yield strengths up to about 78 ksi after heat treating the alloy at 870.degree. C. for one hour. Also, additions of columbium and cerium were made to these alloys and were reported to provide an increase in tensile strengths but yielded room-temperature ductilities of only up to 14%. In another technical article entitled "The Mechanical Properties of Iron-Aluminum Alloys" by W. Justusson et al, Transactions of the ASM, Vol. 49, pp 905-923, 1957, several variables affecting the ductility of iron-aluminum alloys were examined. This article reported on room temperature mechanical properties of Fe-Al alloys containing up to 16% aluminum and such alloys containing carbon and carbon plus titanium. The alloys were hot worked, annealed at 1400.degree. F. for one hour and furnace cooled or water quenched. This investigation revealed that the ductility decreased while the tensile strength and the yield strength of the furnace cooled alloys increased in alloys with increasing aluminum content and that a sharp decrease in ductility occurs in alloys with an aluminum content between 8 and 10%. A comparison in this article of the differences in the elongation of water quenched and oil quenched alloys containing 14% aluminum showed that the oil quenched alloys had higher ductility over a range of quenching temperatures. The yield strength and elongation of an alloy containing 11% aluminum were also determined over a range of quenching temperatures which illustrated that the elongation of the alloy rapidly increased while the yield strength of the alloy rapidly decreased with quenching temperatures between about 482.degree. to 649.degree. C. The authors in this article also reported that in alloys with less than 10% weight percent aluminum and carbon up to several weight percent the ductility of the alloys was retained at 10 to 15% regardless of heat treatment but that the oxidation resistance was seriously affected.
A more recent effort to increase room-temperature ductility and reduce hydrogen embrittlement of iron-aluminum alloys in or near the Fe.sub.3 Al compositions is described in commonly assigned U.S. Pat. No. 5,084,019, issued Jan. 12, 1992 and incorporated herein by reference. This patent is directed to thermomechanically working the alloys to produce an elongated grain structure in the alloys and quenching the alloys at a temperature greater than about 650.degree. C. for providing the worked alloys with a B2-type ordered structure. These alloys, when heat-treated at 700.degree. C. followed by an oil quench, provided room-temperature ductility approaching 20% with both yield strength and ultimate tensile strength increasing with increasing ductility. Also, because of elongated grain structure, a reduced number of grain boundaries were present in the direction transverse to the working direction so as to significantly reduce diffusion paths for the hydrogen into the alloy for reducing hydrogen embrittlement.
The aforementioned and other efforts previously employed for improving properties of Fe-Al alloys of various aluminum concentrations including alloys incorporating selected alloy elements, provided marked improvements in properties of the Fe-Al alloys including improvements in room-temperature ductility, ultimate tensile strength, yield strength, and tensile elongation and a reduction in hydrogen embrittlement. However, even with these improvements there was still a need for providing even greater improvements in the properties of Fe-Al alloys, particularly in the areas of greater room-temperature ductility, greater tensile strength and yield strength at temperatures greater than about 600.degree. C., increased resistance to oxidation, aqueous corrosion pitting and sulfidation, and the minimization of environmental embrittlement so as to provide Fe-Al alloys with properties which will permit their use in an enhanced range of structural applications, many of which now require the use of relatively expensive and heavier stainless steels.